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Journal of Neuroscience, Vol 4, 2725-2737, Copyright © 1984 by Society for Neuroscience
Automatic gain control in the bat's sonar receiver and the neuroethology of echolocation
SA Kick and JA Simmons
The sensitivity of the echolocating bat, Eptesicus fuscus, to sonar echoes
at different time delays after sonar emissions was measured in a two-choice
echo detection experiment. Since echo delay is perceptually equivalent to
target range, the experiment effectively measured sensitivity to targets at
different ranges. The bat's threshold for detecting sonar echoes at a short
delay of only 1.0 msec after emissions (corresponding to a range of 17 cm)
was 36 dB SPL (peak to peak), but the threshold decreased to 8 dB SPL at a
longer delay of 6.4 msec (a range of 1.1 m). Prior research has shown that,
at even longer delays (corresponding to ranges of 3 to 5 m), the bat's
threshold is in the region of 0 dB SPL. Contractions of the bat's middle
ear muscles synchronized with the production of echolocation sounds cause a
transient loss in hearing sensitivity which appears to account for the
observed echo detection threshold shifts. The bat's echo detection
thresholds increase by approximately 11 dB for each reduction in target
range by a factor of 2 over the span from 17 cm to 1.1 m. As range
shortens, the amplitude of echoes from small targets also increases, by 12
dB for each 2-fold reduction in range. Thus, when approaching a target, the
bat compensates for changes in echo strength as target range shortens by
changing its hearing threshold. Since this compensation appears to occur in
the middle ear, the bat regulates echoes reaching the cochlea to a stable
amplitude during its approach to a target such as a flying insect. In
addition to this automatic gain control linked to target range, the bat
aims its head to track a target's position during approach, thus
stabilizing echo amplitude even if the target's direction changes. We
hypothesize that the bat's directional emissions, directional hearing,
middle ear muscle contractions, and head aim response collectively create a
three- dimensional spatial tracking filter which the bat locks onto targets
to stabilize echo amplitudes during interception of prey. We further
hypothesize that this regulation, which cancels echo amplitude changes
caused by the target's changing spatial position, leaves the bat free to
observe echo amplitude changes caused by the target's own actions, such as
insect wing beats. Elimination of spatially dependent echo amplitude
changes removes the cause of potentially troublesome changes in neural
response latency and keeps stimulation from echoes in the "tip" region of
auditory nerve fiber tuning curves.(ABSTRACT TRUNCATED AT 400 WORDS)
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